Conventional filled polymer systems often have improved modulus, stiffness, and hardness relative to unfilled polymer systems. Use of nanofillers in polymers can improve the creep-resistance, wear-resistance, and modulus of the nanocomposite, without adversely affecting polymer aesthetics like clarity. Nanoparticles can also have a strong influence on the glass transition temperature (Tg) of polymers.
Although the high surface area of nanoparticles creates a large interface with host polymers, this high surface area also makes nanoparticles more prone to forming larger particles through agglomeration (a potentially reversible self-association that is frequently difficult and/or costly to reverse) or aggregation (an irreversible self-association). Agglomerated and aggregated nanoparticles frequently do not offer the level of benefits afforded by well-dispersed primary nanoparticles because they have less surface area in contact with the polymer matrix.
Colloidal silica is a potentially convenient source of nanoparticles (particles that are 100 nm in diameter or smaller) that might be blended with a polymer to improve various physical properties of the polymer. But colloidal silica can be difficult to disperse in solvents or polymers because the polar silanol groups on the surface of the nanoparticles can cause them to agglomerate. Even worse, the silanols can react chemically with each other (“condense”) and form irreversible linkages that cause the particles to irreversibly aggregate.
Attempts to overcome this tendency to agglomerate have included grafting polystyrene “brushes” onto the silica nanoparticle surface, but these modified particles are useful only for blends of polymers of the same composition as the brushes, namely polystyrene. In addition, this approach uses an expensive multistep, reversible addition-fragmentation chain transfer polymerization process to modify the surface.
Silanes can also be used to modify silica surfaces like glass, glass fibers, and fumed silica (aggregates of silica nanoparticles), but are rarely used with primary, unaggregated silica particles. Phenylsilane modification improves the compatibility and dispersibility of silica nanoparticles in non-polar aromatic polymers such as polystyrene. Similarly, perfluoroalkylethylsilanes can be used for fluoropolymers.
Surface modification of colloidal silica (unaggregated silica nanoparticles suspended in a liquid medium) is not as facile as surface modification of glass or aggregated particles. The modification can adversely affect the stability of the nanoparticles and cause them to agglomerate or irreversibly aggregate, which leads to particle clusters that are not nanoparticles. This agglomeration or aggregation can also make the particles settle out or form a gel. These suspended particle clusters, settled particles, or gels cannot usually be well-dispersed in polymers.
It is possible to modify silica with a few selected silane reagents without these adverse effects. However, silane modification of silica is slow. To accelerate the surface modification reaction and increase the degree of modification of the silica particles in colloidal and non-colloidal form, heat can be employed. However, when colloidal silica nanoparticles are suspended in low-boiling solvents like 2-propanol or 2-butanone, the reactions must be carried out at elevated pressures as well, since a temperature that is sufficient to effect the modification to a sufficient degree of completion in an economical length of time is above the solvents' boiling points. While it is possible to find commercially available colloidal silica products in solvents of higher boiling point, these solvents may not be compatible with the polymer in which the particles are to be dispersed by solution blending. In solution blending, it is necessary to dissolve the particle and polymer in the same solvent, or in solvents which are miscible with each other. It may not be economical or technically feasible to transfer the silica nanoparticles from their commercially available colloids to a higher boiling point solvent that is compatible with the polymer.
When elevated temperature cannot be used, it is desirable to find a catalyst to accelerate the surface modification process. The silane modification of glass surfaces (a non-colloidal form of silica) is slow and is therefore sometimes carried out with acid catalysts, and occasionally with amine catalysts. Although catalysts can be used for modification of non-colloidal silica (e.g., glass or fumed silica), they may cause the nanoparticles in colloidal silica to agglomerate or aggregate into large clusters or to undesirably settle out of suspension or form a gel.
It has been found that aromatic aminosilanes do surface-modify colloidal silica without causing the silica nanoparticles to gel, agglomerate, or aggregate, but the reactions are very slow and can be incomplete if not carried out for a very long time. Thus, there is a need to increase the rate of this surface modification by aromatic aminosilanes for colloidal silica with nanoparticles at temperatures at or below the boiling point of the solvent in which the silica is suspended, without causing the silica nanoparticles to gel, agglomerate, or aggregate. There is also a need to increase the rate of surface modification of colloidal silica with nanoparticles by other silanes that react slowly with colloidal silica at temperatures at or below the boiling point of the solvent in which the silica is suspended.